Recommends

Comments

Comments on Primary Papers and News

In the Cell paper by Zwilling et al. on oral kynurenine 3-monooxygenase (KMO) inhibition in Huntington's and Alzheimer's transgenic mouse models, a striking effect was the prevention of pre-synaptic protein loss in the transgenic models of AD (APP-Tg mice) and HD (R6/2 mice). In these models, systemic (oral) administration using the small-molecule KMO inhibitor JM6 increased brain levels of kynurenic acid, prevented behavioral deficits in APP-Tg mice, and increased survival and reduced CNS microglial activation in R6/2 mice. Importantly, in both models, JM6 prevented the loss of the pre-synaptic protein synaptophysin. Recent reports (e.g., Chung et al., 2009) have highlighted synaptic changes as an early pre-degenerative event in neurodegenerative diseases, and such changes are a target for pre-symptomatic neuroprotective interventions. The striking effect of JM6 on synaptic changes and other degenerative alterations in these two in vivo models are encouraging for future protective therapies for several neurodegenerative diseases.

This is an interesting study that examined the neuroprotective efficacy of JM6, a small molecule inhibitor of kynurenine 3-monooxygenase (KMO). The authors demonstrate that oral administration of JM6 inhibits KMO in the blood, resulting in an increase in kynurenic acid levels. Interestingly, they demonstrate that JM6 does not cross the blood-brain barrier, and does not inhibit KMO in the brain, yet there are neuroprotective effects in transgenic mouse models of AD and of HD. The inhibition of KMO in the blood results in an increase in kynurenine, which then leads to an increase in kynurenic acid concentrations both in blood as well as in the brain. In the brain, the increase in kynurenic acid is thought to exert neuroprotective effects by blocking both AMPA and NMDA excitatory amino acid receptors, and by blocking the pre-synaptic α7 nicotinic acetylcholine receptors, resulting in a reduction in glutamate release and excitotoxicity.

In APP transgenic mice, JM6-increased brain kynurenic acid levels prevent synapse loss and spatial memory loss in the Morris water maze. The authors also carried out studies in the R6/2 transgenic mouse model of HD, using both a high or a low dose of JM6 administered orally. These studies demonstrated that there was a significant increase in survival, with either dose level of JM6, as well as protection against loss of synaptophysin immunoreactivity in the striatum. There was a significant reduction in the number of neurons immunoreactive for the calcium regulated immediate early gene c-Fos, and a reduction in IBA1-positive activated microglia, but no alterations in huntingtin (Htt) inclusions. This is consistent with a number of other studies, which have shown that Htt inclusions do not correlate with neuroprotection or improved survival.

Overall, these are novel and interesting findings, which show that manipulation of the kynurenine pathway in the periphery, leading to increased kynurenine and kynurenic acid levels in the brain, produces neuroprotective and behavioral improvement in transgenic mouse models of AD and HD. In the HD mice, the improvement in survival was several weeks, which is consistent with other compounds that block excitatory amino acid receptors, such as remacemide and memantine. These results are also consistent with our studies in which we administered kynurenine in combination with probenecid, an inhibitor of organic acid transport, which increases kynurenic acid concentrations in the brain and exerts protective effects against pentylentetrazol and NMDA-induced seizures (Vecsei et al., 1992; Miller et al., 1992). Other studies showed that administration of L-kynurenine and probenecid, produced dose-dependent attenuation of quinolinic acid striatal lesions (Harris et al., 1998). We also demonstrated that administration of L-kynurenine alone, or in combination with probenecid, produced dose-dependent neuroprotection against hypoxia-ischemia and NMDA lesions in neonatal rats (Nozak and Beal, 1992).

We and others previously showed that there were significant reductions in kynurenic acid concentrations in HD postmortem brain tissue, which may increase the vulnerability of neurons to excitotoxic cell death (Beal et al., 1992; Jauch et al., 1995). The present approach is a novel one in that they blocked KMO activity in the periphery, which results in increased kynurenine, which is transported across the blood-brain barrier and increases kynurenic acid levels in the central nervous system, reducing release of extracellular glutamate as assessed using microdialysis. Administration of probenecid to a transgenic mouse model of HD increased cortical kynurenic acid levels fourfold and increased survival by 35 percent (Vamos et al., 2009). Manipulations of the kynurenine pathway to increase kynurenic acid levels are also protective in a Drosophila model of Huntington’s disease (Campesan et al., 2011). This is a particularly attractive approach, since one is manipulating an endogenous pathway to exert neuroprotective effects, which may be well tolerated without the behavioral toxicity associated with other excitatory amino acid receptor antagonists.

The primary outcome data from the reaZin study appear to be consistent with the proposed action of the intervention. The three cognitive and functional measures used for the series of secondary outcomes are appropriate, but the small size of the sample means that the study was underpowered with respect to any clinical outcome measures. The small sample size was probably responsible also for the lack of balance in baseline measures across the randomized groups. Whether one should see the preliminary clinical outcome results as encouraging is a matter of judgment. The poster presentation does not make it clear whether the composite outcome was specified a priori. If not, the meaning of the p-value of 0.15 is hard to discern. In any event, I cannot agree with the authors' conclusion that these results provide a "strong trend toward cognitive benefit favoring the treatment group."